Well known is the requirement of having available industrial processes for preparing 1,1,1-trifluoro-2,2-dichloroethane with good yields.
It is well known how to prepare 1,1,1-trifluoro-2,2-dichloroethane starting from perchloroethylene in the presence of catalysts based on Cr.sub.2 O.sub.3. See for example Italian patent application No. 24689 A/78 and U.S. Pat. No. 3,755,477.
The process described in U.S. Pat. No. 3,755,477 leads to yields of the mentioned product equal to about 20% and it cannot be utilized on a commercial scale because of the too high amount of by-products.
An alternative process (U.S. Pat. No. 4,145,368 and U.S. Pat. No. 4,192,322) for preparing the compound of the invention comprises reacting 1,1,1-trifluoro-2-chloroethane with chlorine. The yields of the compound of the invention are very low. However, in order to increase the useful product yields, it is possible, according to the above-cited patents, to react the obtained reaction mixture with further starting product 1,1,1-trifluoro-2-chloroethane in the presence of catalysts such as Cr.sub.2 O.sub.3, oxyfluorides or activated carbon.
Also when using this method, the yields of useful product 1,1,1-trifluoro-2,2-dichloroethane are of the order of 14%.
This process is not utilizable on an industrial scale as it requires two reaction steps plus a third step, which is necessary to prepare the starting product. Furthermore, the contact time of the reagents used in the step where the catalyst based on Cr.sub.2 O.sub.3 or the ones cited above is of the order of 90 seconds. These times are too long for an industrial process in the gas phase, because it requires too large reactors.
Processes for the liquid-phase-hydrofluorination of halogenated olefins for preparing chlorofluorohydrocarbons are well known in the art, such processes using, as catalysts, for example TaF.sub.5, BF.sub.3, SbCl.sub.3 F.sub.2.
When perchloroethylene is utilized as a starting product, it is not possible to obtain the product of the invention.
This product is obtainable with very low yields, of about 10%, by previously reacting perchloroethylene with HF and by subsequent hydrofluorination in the presence of TaF.sub.5. See, for example, Journal Fluorine Chemistry, 13 (1979), 7-18, "Chemistry of Hydrogen Fluoride v. Catalysts for Reaction of HF with Halogenated Olefins", A. E. Feiring.
In the art there are known many modifications of the various types of fluorination catalysts which are utilized for preparing perhalo-compounds of chlorofluorohydrocarbons.
There are known, in fact, fluorination catalysts based on chrome oxides, oxyfluorides, fluorides, or copper, iron, nickel, manganese, cobalt fluorides, etc. carried on Al.sub.2 O.sub.3, AlF.sub.3, activated carbon, CaF.sub.2.
However it is well known that carriers different from the fluorinated ones, in the presence of HF tend to give the corresponding fluorinated carriers. Therefore in practice, the reaction which leads to fluorocarbons is carried out in the presence of a fluorinated carrier.
However, the product of the invention is not exemplified in any patent, except the ones mentioned above.
U.S. Pat. No. 3,258,500 described various fluorination catalysts, among which is Cr.sub.2 O.sub.3. The 1,1,1-trifluoro-2,2-dichloroethane yields are of about 16% if it is operated at about 300.degree. C. and they sink to about 3.5% if it is operated at 400.degree. C. In said patent, column 7, lines 30-52, it is specified that it is essential, when using these catalysts, to operate at exactly defined temperatures to obtain good conversions to the desired useful product. As mentioned above, the best mode of this patent for the product 1,1,1-trifluoro-2,2-dichloroethane indicates 16% as the best result.
Furthermore, this patent indicates, as a possible alternative, the possibility of supporting chrome oxide on various inert supports; for example Al.sub.2 O.sub.3 (column 6, lines 23-54). Nevertheless the results so obtained are of little interest as the yields decrease.
The process according to said patent is in any case of low industrial value because the by-product amount is too high.